1. Field of the Invention
The present invention relates to a liquid crystal display device and an electronic apparatus. The present invention particularly relates to a transflective reflective liquid crystal display device which can perform display with sufficient brightness in transparent mode.
2. Description of Related Art
A reflective liquid crystal display device does not have a light source such as a back light and consumes less electricity. Thus, a reflective liquid crystal display device is mainly used in an auxiliary displaying section for various mobile electronic apparatus and device conventionally. However, in the reflective liquid crystal display device displays by using an external light such as available light and light supplied from a lighting device, there was a problem in understanding what is displayed in a dark environment. From that point of view, a liquid crystal display device in which external light is used like an ordinary reflective liquid crystal display device in a light place, and display is visible in darkness by using an internal light source, is proposed. This kind of liquid crystal display device employs a displaying method compatible with a reflective method and a transparent method, and bright display operation and reduction of energy consumption can be realized by changing the display mode between reflective mode and transparent mode according to the circumstance even if the liquid crystal display is used in a dark condition. Hereinafter in this specification, this kind of liquid crystal display device is called a “transflective reflective liquid crystal display device”.
For a transflective reflective liquid crystal display device, a liquid crystal display device provided with a reflective layer made of a metal foil such as aluminum on which light transmitting slits are formed on a surface of a lower substrate is proposed. In such a liquid crystal display device, an influence of parallax due to the thickness of the lower substrate is prevented by forming the metal foil on an inner surface of the lower substrate. Particularly in the case in which a color filter is used, occurrence of color mixing is prevented.
FIG. 12 is a cross section partially showing an example of a transflective reflective liquid crystal display device employing a passive matrix method. In a liquid crystal display device 100, liquid crystals 103 are disposed between a pair of substrates such as an upper substrate 102 and a lower substrate 101. Reflective layers 104 and an insulating layer 106 are layered on the lower substrate 101. Furthermore, a stripe form of scanning electrode 108 made of transparent conductive layer such as Indium Tin Oxide (hereinafter called ITO) is formed on the lower substrate 101, and a orientation layer 107 is formed so as to cover the scanning electrode 108. On the other hand, the color filters 109 are formed on the upper substrate 102, and a flat layer 111 is layered on the color filters 109. Signal electrodes 112 made of transparent conductive layer such as ITO are formed on the flat layer 111 so as to be orthogonal to the scanning electrode 108 in stripe form. An orientation layer 113 is formed so as to cover the signal electrode 112. Reflection layers 104 are formed by metal foil such as aluminum, and light transmitting slits 110 are formed per pixel on the reflection layer 104. An incident light from the lower substrate 101 is transmitted through the slits 110; thus, the reflection layer 104 functions as a transflective reflective layer. Also, a forward scattering plate 118, a phase differentiating plate 119 and an upper polarizing plate 114 are formed on the upper substrate 102 in this order from the upper substrate 102 to the outside. A ¼ wavelength plate 115, a lower polarizing plate 116 are formed on the outside of lower substrate 101. Also, a backlight 117 is attached beneath the lower substrate 101.
When the above liquid crystal display device 100 is used in reflection mode in lighted environments, an external light which is incident from above upper substrate 102 is transmitted through the liquid crystals 103 and is reflected on the surface of the reflection layer 104, and is transmitted through the liquid crystals 103 again and is emitted to the upper substrate 102. When the above liquid crystal display device 100 is used in transparent mode in a dark environment, the light emitted from the backlight 117 located beneath the lower substrate 101 is transmitted through the slits 110 of the reflection layer 104, and is transmitted through the liquid crystals 103, and then is emitted to the upper substrate 102. These lights contribute to the displaying operation in each mode.
In the above liquid crystal display device 100, what is displayed is visible regardless to whether or not there is light. However, there was a problem in which lighting of the display was not sufficient in transparent mode comparing to the reflection mode. This occurs mainly because only the light which is transmitted through the slits 110 of the reflecting layer 104 contributed to the display operation in the transparent mode, and the light was lost in the ¼ wavelength plate 115 and the polarizing plate 116 formed on an outer surface of the lower substrate 101.
In a liquid crystal display device 100 shown in FIG. 9, when display operation is performed in a transparent mode, a light emitted from a backlight 117 is incident on a liquid crystal display device unit from outside of a lower substrate 101, and the light which is transmitted through the slits 110 among the above light contributes to display operation. Here, in order to perform a dark display in a liquid crystal display device 100, the light which runs toward the upper substrate 102 from the slits 110 must be a circular polarized light. Therefore, the light which is emitted from the backlight 117 and is transmitted through the slits 110 also must be a circular polarized light; thus, ¼ wavelength plate 115 for converting light which is transmitted through a lower polarizing plate 116 and is converted to a linear polarized light into a circular polarized light is necessary.
Next, among lights emitted from the backlight 117, a light which is not transmitted through the slits 110 is emitted from the backlight 117 and is transmitted through the lower polarizing plate 116 and is converted into a linear polarized light which is parallel with the paper and is transmitted through the ¼ wavelength plate 115 and becomes a circular polarized light and then reaches a reflection layer 104. When this light is not incident on the slits 110 and is reflected by a surface close to the lower substrate 101 of the reflecting layer 104, this light becomes a circular polarized light having a counter-rotation direction to the circular polarized light which is incident on the reflection layer 104. When this light is transmitted through the ¼ wavelength plate 115, this light is converted to a linear light which is orthogonal to the surface of the drawing. Therefore, this light is absorbed by the lower polarizing boar 116 having a transparent axis which is parallel to the surface of the drawing. That is, among lights emitted from the backlight 117, all the light which is not transmitted through the slits 110 and is reflected on a surface close to a back side of the reflecting layer 104 is absorbed by the lower polarizing plate 116 of the lower substrate 101.
Furthermore, with regard to a case in which a light display is performed in transparent mode in a liquid crystal display device shown in FIG. 12, a light which is transmitted through the slits 110 and is incident on liquid crystals 103 is transmitted through an upper polarizing plate 114 of an upper substrate 102 without being influenced by liquid crystals 103 and is emitted in an upper direction of the liquid crystal display device. However, because the light which travels toward the upper substrate 102 from the slits 110 becomes a circular polarized light because of ¼ wavelength plate 115, almost half the light which travels toward the upper substrate 102 from the slits 110 is absorbed by the upper polarizing plate 114 when being transmitted through the upper polarizing plate 114 having a transparent axis which is parallel with the surface of the drawing.
Due to the above reasons, in the above liquid crystal display device 100, it was not possible to illuminate the display to be brighter in a transparent mode. In order to solve the above problem, a liquid crystal display device having a structure shown in FIG. 13 is proposed. In a liquid crystal display device 200 shown in FIG. 13, liquid crystals 203 are put between a pair of transparent lower substrate 201 and upper substrate 202, a reflecting polarizing layer 204 and an insulating layer 206 are layered on a lower substrate 201, and a scanning electrode 208 made of a transparent conductive layer such as ITO in stripe form is formed thereon, and an orientation layer 207 is formed so as to cover the scanning electrode 208. On the other hand, color filters 209 are formed on an inner surface of the upper substrate 202, and a flat layer 211 is layered thereon. On the flat layer 211, signal electrodes 212 made of a transparent conductive layer such as ITO are formed in stripe form so as to be in an orthogonal direction to the scanning electrode 212. The orientation layer 213 is formed so as to cover the signal electrodes 212. Fine aperture sections having almost 50 nm width of a reflecting polarizing layer 204 are made of a metal layer such as aluminum in a slit form with intervals of 100 nm to 400 nm. Among incident lights to the reflecting polarizing layer 204, polarized light which is parallel with a slit aperture section is reflected, and a polarized light which is orthogonal to the aperture section is transmitted therethrough. On an outer surface of the upper substrate 202, a forward scattering plate 218, a phase differentiating plate 219, and an upper polarizing plate 214 are disposed in this order from the upper substrate 202 in the outer direction. Also, a backlight 217 is disposed beneath the lower substrate 201.
In a liquid crystal display device 200 having the above structure, the light which is incident to the upper polarizing plate 214 is a linear polarized light which is different from a circular polarized light in transparent mode. Such a property is different from the case of a liquid crystal display device 100 shown in FIG. 12; therefore, a liquid crystal display device 200 can illuminate the display in transparent mode compared to the case of a liquid crystal display device 100. Also, the light which is reflected without being transmitted through the reflecting polarizing layer 204 returns to the backlight 217, and during the repetition of the reflection between the reflecting polarizing layer 204 and the backlight 217, the polarization state of such light changes so as to be able to be transmitted through the reflecting polarizing layer 204. Thus, the light of the backlight 217 can be used more effectively than in the case of above liquid crystal display device 100.
However, in the transparent mode of a liquid crystal display device 200, when an external light is incident on the liquid crystal display device 200, displaying contrast of the liquid crystal display device 200 decreases remarkably, and the display becomes invisible sometimes according to the intensity of the external light.